Colicin E3

Colicin E3 is a type of Colicin, a 60kDa bacteriocin made by E. coli which acts against other nearby E. coli to kill them with its 16s rRNase activity; it digests the 16s ribosomal subunit, ultimately leading to the death of the cell.

Synthesis and release
The plasmid containing the colicin E3 gene is called ColE3-CA38, which is structurally homologous to that of the Colicin E2 plasmid. The expression of the operon is regulated by the lexA protein, as behind the promotor there are two inverted lexA binding regions. The colicin, alongside its immunity protein, are not produced as a precursor protein requiring modification post-translationally. Instead they leave the colicinergic cell by a non-specific mechanism, whereby the permeability of the cell is increased.

When colicin E3 is synthesised, it binds its 10kDa Colicin Immunity Protein, Im3, to its cytotoxic domain in the cytoplasm. This prevents its RNase activity from occurring in the colicinogenic host, so it does not kill it. It is released as this complex into the surrounding area, and Im3 is only released upon binding to a target cell. In the normal state, Im3 is produced at a basal level to protect the cell from the colicin that it is producing, but in the SOS-induced state, initiated by lexA binding, a large amount is produced to protect the cell from externally produced ColE3.

Mechanism of uptake
ColE3 initially binds to the BtuB vitamin B12 receptor. Formation of this complex leads to the unfolding of the N terminal receptor binding coiled-coil domain of ColE3. The structure shows the complex formed between BtuB and the ColE3 translocation domain. Cells with a mutation in the gene encoding the BtuB gene show immunity to ColE3 attack, as well as other Colicins that use the same receptor.

After binding to the BtuB receptor, the complex recruits OmpF, and a group of proteins from the Tol group of proteins, TolQRAB. The mechanism by which these proteins aid the translocation of ColE3 is as yet unknown, but the colicin then traverses the outer membrane and reaches the periplasm.

For more information about the uptake of Colicin E3 see the uptake section on Colicin E9, which has a similar mechanism.

Killing Activities
The C terminal domain of colicin E3 kills the cells that it penetrates with its 16s rRNase activity, by actively degrading the 16s rRNA subunit of the cell's 70S ribosome. This prevents the cell from producing any proteins once attacked, which will lead to the death of the cell. This rRNA cleavage is specific in vivo, but random when the rRNA domain is isolated in vitro. The cleavage is unaffected by the presence of cellular ribonucleases. The specific cleavage observed in vivo occurs within the ribosomal decoding A-site, in between A1493 and G1494, resulting in the complete halt of protein biosynthesis, leading to cell death.